This invention relates generally to a magnetic current meter for sensing the flow of liquid conducted through an open channel, and more particularly to a meter of this type which produces a mean flow velocity signal that is substantially independent of liquid level fluctuations in the channel.
In order to measure the flow of water or other liquid conducted through an open channel, it is necessary to determine the level of water in the channel as well as the velocity of flow. The cross-sectional area of the flow is normal to the flow direction, and this area is therefore a function of water level. The flow rate of the water through the open channel is obtained by multiplying the mean flow velocity by the cross-sectional area of the flow. To determine the flow velocity, use is generally made of a magnetic current meter.
The sensor of a conventional magnetic current meter includes a pipe of insulating material within which is coaxially disposed a core rod having a coil wound thereon, the coil being surrounded by an electrostatic shield. The remaining free space in the pipe is stuffed with an insulating filler. A pair of button-shaped electrodes are mounted at diametrically-opposed positions on the outer surface of the pipe.
When an alternating-current is supplied to the coil of the sensor, an electromagnetic field is established outside of the pipe whose lines of flux pass in a direction parallel to the axis of the pipe and perpendicular to the diametrical line joining the button electrodes. When the flowing water intersects the magnetic flux of the magnet current meter, a voltage signal proportional to flow velocity is induced in the electrodes in accordance with Faraday's law.
While one can detect the flow velocity with a conventional magnetic current meter in the immediate vicinity of the sensor, it is difficult to determine the mean flow velocity, for the liquid flow through an open channel has a specific flow profile which depends on the configuration of the channel or on the manner in which the channel is constructed. Moreover, the flow profile changes with water level fluctuations.
In a conventional magnetic current meter making use of button electrodes, the voltage signal representing flow velocity that is derived from these electrodes depends on the disposition of the meter sensor. In order, therefore, to obtain a signal which accurately reflects the mean flow velocity, compensation means are required in conjunction with the sensor. That is to say, to derive a mean flow velocity signal with a conventional magnetic current meter positioned at a fixed water level, one must compensate the flow velocity signal produced by the meter with a water level signal.
To this end, when first installing a conventional magnetic current meter in a channel, one must investigate from time to time the relationship between the water level, the flow velocity at the measuring point and the mean flow velocity. And the function generator which is associated with the meter to obtain the mean flow velocity signal must be configured to satisfy this relationship. The drawback to this meter compensation approach is that the more complex the relationship, the less accurate is the compensation.